Preheat the oven to 190°C, fan 170°C, gas 5. Trim the courgettes and cut into small pieces. Cut the peppers in half, deseed, and cut into small pieces. Peel the red onion, cut in half, then cut into small pieces. Put all of the vegetables, garlic and rosemary (if using) with the chicken pieces in a roasting tin. Drizzle with a little olive oil and roast for 35 mins or until the chicken is cooked through. You might like to stir the vegetables once to help them cook evenly.
Meanwhile, put the couscous into a bowl and pour over 250ml boiling water. Rest a plate over the top and leave until the couscous has absorbed the water.
Cut the cooked chicken into small pieces.
Fluff up the couscous with a fork, squeeze the garlic out of its papery skin and add to the couscous with the roasted vegetables and chopped chicken. Scatter over the fresh chopped mint, pour over the orange juice and mix everything together.